The present invention relates to polyester blend compositions. More specifically, the invention relates to a polyester blend composition of two or more copolymers, particularly polyester blends of one or more branched biodegradable copolymers with one or more linear biodegradable copolymers, a biodegradable film formed from the blend composition, and a method for manufacturing the blend composition and film.
Prompted by societal concerns and demands concerning the disposal of plastic products such as plastic films, research is being done on biodegradable resin compositions. Active efforts are underway to develop biodegradable aromatic polyester resin compositions that are degradable under the high-humidity, high-temperature conditions associated with waste composting processes. Various uses are being proposed for such compositions. For example, U.S. Pat. No. 5,053,482 to Tietz, U.S. Pat. No. 5,097,004 to Gallagher et al., and U.S. Pat. No. 5,097,005 to Tietz describe polyesters prepared by polymerizing a glycol component made of ethylene glycol and diethylene glycol with two acid components; namely, an alkali metal or alkaline earth metal salt of a sulfonic acid, and terephthalic acid. These patents also describe fibers, films, sheets, and fiber nonwoven fabrics composed of such polyesters.
U.S. Pat. No. 5,443,780 to Matsumoto et al. and U.S. Pat. No. 5,076,983 to Loomis et al. describe biodegradable films and methods for their production, which films are biaxial films produced by melt-extruding a biodegradable polylactic acid polymer as a film and then orienting the extruded film. Such films have properties similar to those of films endowed with good strength and clarity, such as polypropylene film, oriented polystyrene film, and oriented polyethylene terephthalate film.
U.S. Pat. No. 5,076,983 to Loomis et al. discloses a film of improved properties, including strength, stiffness and clarity that is formed by extruding a polyhydroxy acid composition and stretching the extruded composition to produce a biaxially oriented film.
Films made of ordinary resins such as polyolefin resins, polyamide resins, polyester resins, and polyvinyl chloride resins are currently widely used today as packaging materials.
When films formed of degradable polymer are used as packaging materials and in related applications, the relevant film properties (e.g., mechanical strength, clarity, and gas barrier properties) generally fall somewhat short of the properties of resins that have hitherto been used in packaging materials, such as polyolefin resins, polyamide resins, polyester resins, and polyvinyl chloride resins.
As noted above, oriented films made of polylactic acid resins have sufficient strength and clarity. These films, however, lack sufficient stiffness, impact resistance, heat-sealability, and gas barrier properties for practical use.
Moreover, when a film is extruded using polylactic acid, and especially poly(L-lactic acid), the melting point is 170° C. Because the temperature difference between the glass transition temperature (about 60° C.) and the crystallization temperature (about 110° C.) is small, the allowable range of conditions for carrying out orientation is narrow. As a result, film composed of this type of resin is poorly suitable for orientation.
To overcome these shortcomings, International Publication Number WO01/10928 describes an oriented aromatic polyester film formed by an aromatic polyester copolymer consisting of repeating units derived from terephthalic acid, a metal salt of sulfonic acid, aliphatic dicarboxylic acid, ethylene glycol, and diethylene glycol, wherein the melting point and glass transition temperature of the film is controlled to provide enhanced performance properties, particularly sealing strength.
It is desirable to provide biodegradable compositions that have enhanced performance properties, such as softness and tear strength, and that are suitable for use in such applications as packaging materials.
All references cited herein are incorporated by reference.